last_will

last_will names the message the server publishes when the connection ends without a clean disconnect, such as a service's offline status.

The will travels in the CONNECT packet, and the server keeps it for as long as the connection lasts. It has a topic, a payload, a QoS and a retain flag, like any publish. The payload is bytes, written as the service's readers decode them; no codec runs on it.

The server publishes the will when the connection ends any other way than by the client's DISCONNECT. That covers a process that dies, a network that drops, a connection silent past its keep-alive, and a second client that takes the client id over. A clean shutdown sends a DISCONNECT, and the server discards the will. A retained will stays on its topic, so a dashboard that subscribes later still reads that the service is gone.

The server publishes the will, so a TestApp test in process runs the app without it. A live test shows it. The generated AsyncAPI document reports the will's topic, QoS and retain flag, not its payload.

Example

Each instance announces its own death on services/<client id>/status, retained.

Macros
Manual

examples/mqtt_broker_last_will.rs
/// If the connection ends without a clean disconnect, the server publishes `{"online":false}`
/// on the service's status topic and keeps it there for every later subscriber.
fn broker(url: &str, client_id: &str) -> MqttBroker {
    MqttBroker::new(url, client_id).last_will(
        format!("services/{client_id}/status"),
        r#"{"online":false}"#,
        Qos::AtLeastOnce,
        true,
    )
}

#[ruststream::app]
fn app() -> impl App<State = ()> {
    service(broker("mqtt://localhost:1883", "telemetry-svc"))
}

examples/manual/mqtt_broker_last_will.rs
/// If the connection ends without a clean disconnect, the server publishes `{"online":false}`
/// on the service's status topic and keeps it there for every later subscriber.
fn broker(url: &str, client_id: &str) -> MqttBroker {
    MqttBroker::new(url, client_id).last_will(
        format!("services/{client_id}/status"),
        r#"{"online":false}"#,
        Qos::AtLeastOnce,
        true,
    )
}

fn app() -> impl App<State = ()> {
    service(broker("mqtt://localhost:1883", "telemetry-svc"))
}

Example​

Example